nixos/nixos-install: add support for --file and --attr flags

nixos/modules/installer/tools/manpages/nixos-install.8

@@ -14,6 +14,8 @@
 .Op Fl -root Ar root
 .Op Fl -system Ar path
 .Op Fl -flake Ar flake-uri
+.Op Fl -file | f Ar path
+.Op Fl -attr | A Ar attrPath
 .Op Fl -impure
 .Op Fl -channel Ar channel
 .Op Fl -no-channel-copy
@@ -111,6 +113,32 @@ output named
 .Ql nixosConfigurations. Ns Ar name Ns
 \&.
 .
+.It Fl -file Ar path , Fl f Ar path
+Enable and build the NixOS system from the specified file. The file must
+evaluate to an attribute set, and it must contain a valid NixOS configuration
+at attribute
+.Va attrPath Ns
+\&. This is useful for building a NixOS system from a nix file that is not
+a flake or a NixOS configuration module. Attribute set a with valid NixOS
+configuration can be made using
+.Va nixos
+function in nixpkgs or importing and calling
+.Pa nixos/lib/eval-config.nix
+from nixpkgs. If specified without
+.Fl -attr
+option, builds the configuration from the top-level
+attribute of the file.
+.
+.It Fl -attr Ar attrPath , Fl A Ar attrPath
+Enable and build the NixOS system from nix file and use the specified attribute
+path from file specified by the
+.Fl -file
+option. If specified without
+.Fl -file
+option, uses
+.Va [root] Ns Pa /etc/nixos/default.nix Ns
+\&.
+.
 .It Fl -channel Ar channel
 If this option is provided, do not copy the current
 .Dq nixos

nixos/modules/installer/tools/nixos-install.sh

@@ -17,6 +17,9 @@ mountPoint=/mnt
 channelPath=
 system=
 verbosity=()
+attr=
+buildFile=
+buildingAttribute=1
 
 while [ "$#" -gt 0 ]; do
     i="$1"; shift 1
@@ -41,6 +44,24 @@ while [ "$#" -gt 0 ]; do
           flakeFlags=(--experimental-features 'nix-command flakes')
           shift 1
           ;;
+        --file|-f)
+            if [ -z "$1" ]; then
+                log "$0: '$i' requires an argument"
+                exit 1
+            fi
+            buildFile="$1"
+            buildingAttribute=
+            shift 1
+            ;;
+        --attr|-A)
+            if [ -z "$1" ]; then
+                log "$0: '$i' requires an argument"
+                exit 1
+            fi
+            attr="$1"
+            buildingAttribute=
+            shift 1
+            ;;
         --recreate-lock-file|--no-update-lock-file|--no-write-lock-file|--no-registries|--commit-lock-file)
           lockFlags+=("$i")
           ;;
@@ -101,17 +122,30 @@ while [[ "$checkPath" != "/" ]]; do
     checkPath="$(dirname "$checkPath")"
 done
 
-# Get the path of the NixOS configuration file.
-if [[ -z $NIXOS_CONFIG ]]; then
-    NIXOS_CONFIG=$mountPoint/etc/nixos/configuration.nix
-fi
-
-if [[ ${NIXOS_CONFIG:0:1} != / ]]; then
-    echo "$0: \$NIXOS_CONFIG is not an absolute path"
+# Verify that user is not trying to use attribute building and flake
+# at the same time
+if [[ -z $buildingAttribute && -n $flake ]]; then
+    echo "$0: '--flake' cannot be used with '--file' or '--attr'"
     exit 1
 fi
 
-if [[ -n $flake ]]; then
+# Get the path of the NixOS configuration file.
+if [[ -z $flake && -n $buildingAttribute ]]; then
+    if [[ -z $NIXOS_CONFIG ]]; then
+        NIXOS_CONFIG=$mountPoint/etc/nixos/configuration.nix
+    fi
+
+    if [[ ${NIXOS_CONFIG:0:1} != / ]]; then
+        echo "$0: \$NIXOS_CONFIG is not an absolute path"
+        exit 1
+    fi
+elif [[ -z $buildingAttribute ]]; then
+    if [[ -z $buildFile ]]; then
+        buildFile="$mountPoint/etc/nixos/default.nix"
+    elif [[ -d $buildFile ]]; then
+        buildFile="$buildFile/default.nix"
+    fi
+elif [[ -n $flake ]]; then
     if [[ $flake =~ ^(.*)\#([^\#\"]*)$ ]]; then
        flake="${BASH_REMATCH[1]}"
        flakeAttr="${BASH_REMATCH[2]}"
@@ -129,11 +163,16 @@ if [[ -n $flake ]]; then
     flake=$(nix "${flakeFlags[@]}" flake metadata --json "${extraBuildFlags[@]}" "${lockFlags[@]}" -- "$flake" | jq -r .url)
 fi
 
-if [[ ! -e $NIXOS_CONFIG && -z $system && -z $flake ]]; then
+if [[ ! -e $NIXOS_CONFIG && -z $system && -z $flake && -n $buildingAttribute ]]; then
     echo "configuration file $NIXOS_CONFIG doesn't exist"
     exit 1
 fi
 
+if [[ ! -z $buildingAttribute && -e $buildFile && -z $system ]]; then
+    echo "configuration file $buildFile doesn't exist"
+    exit 1
+fi
+
 # A place to drop temporary stuff.
 tmpdir="$(mktemp -d -p "$mountPoint")"
 trap 'rm -rf $tmpdir' EXIT
@@ -163,11 +202,20 @@ fi
 # Build the system configuration in the target filesystem.
 if [[ -z $system ]]; then
     outLink="$tmpdir/system"
-    if [[ -z $flake ]]; then
+    if [[ -z $flake && -n $buildingAttribute ]]; then
         echo "building the configuration in $NIXOS_CONFIG..."
         nix-build --out-link "$outLink" --store "$mountPoint" "${extraBuildFlags[@]}" \
             --extra-substituters "$sub" \
             '<nixpkgs/nixos>' -A system -I "nixos-config=$NIXOS_CONFIG" "${verbosity[@]}"
+    elif [[ -z $buildingAttribute ]]; then
+        if [[ -n $attr ]]; then
+            echo "building the configuration in $buildFile and attribute $attr..."
+        else
+            echo "building the configuration in $buildFile..."
+        fi
+        nix-build --out-link "$outLink" --store "$mountPoint" "${extraBuildFlags[@]}" \
+            --extra-substituters "$sub" \
+            "$buildFile" -A "${attr:+$attr.}config.system.build.toplevel" "${verbosity[@]}"
     else
         echo "building the flake in $flake..."
         nix "${flakeFlags[@]}" build "$flake#$flakeAttr.config.system.build.toplevel" \